1. Technical Field
This invention relates to a method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid can be detected to determine flow rate. The invention further relates to improving the precision of measurement and in particular, flow sensor stability, and reducing power consumption, complexity and cost.
2. Discussion
Ultrasonic transit time flow sensors, also known as xe2x80x9ctime-of-flightxe2x80x9d ultrasonic flow sensors, detect the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of the flowing fluid and process this information to derive a fluid flow rate. One method used by these sensors and described in the U.S. Pat. No. 4,221,128 to Lawson et al, is to transmit a burst of continuous waves concurrently on the upstream and downstream transducers. The difference in time between the reception of those signals is a measure of flow rate. Lawson et al use a frequency translation means to convert the received frequencies down to a relatively low frequency for detecting the time difference, which requires some elaboration in electronic circuitry. Furthermore, Lawson et al do not incorporate any error detecting means to compensate for the drift of components as might occur over a period of time or temperature range, and rely instead on costly high stability components.
It is therefore an object of the present invention to incorporate an error detecting means to compensate for component errors, as well as acoustic path related errors, thereby enabling lower cost components and simpler circuits to be used.
The present invention satisfies the above and other objects by providing an ultrasonic, transit-time flow sensor in accordance with preferred embodiments of the present invention. The sensor, in a first instance, generally operates to detect a signal corresponding to the phase difference between signals from two transducers during the interval of an acoustic transmission, and using this signal to adjust the output flow rate signal to compensate for circuit error inherent in the flow sensor.
In one of the preferred embodiments of the flow sensor, a burst of an acoustic signal is transmitted from a first transducer located upstream of a second transducer. After a period of time greater than that of the acoustic burst signal duration, the signal is received by the second transducer located downstream of the first transducer. Concurrently with the transmission of the first transducer, the second transducer similarly transmits a burst of acoustic energy which is received by the first transducer. Although the transmitted signals are of the same frequency, they are 90 degrees out of phase.
Each transducer has its own receiver. During the transmitted bursts of acoustic energy, the corresponding electrical signals are also routed from the transducers to the two receivers and also an Exclusive-Or phase detector. The output from the phase detector is filtered, sampled and stored to become a signal representative only of the phase shift of the electrical circuits of the flow sensor.
At the time when the transmitted acoustic signals are expected to be received by the transducers, the transducer signals are again routed into the receivers. The Exclusive-Or phase detector detects the difference in phase between the signals resulting from the difference in propagation transit time of the flowing fluid, in addition to the phase shift due to the electrical circuits of the flow sensor. The phase detector also provides a DC signal which is similarly filtered, sampled and separately stored. With amplification, this signal becomes the basic output flow rate signal.
The phase detected signal acquired during the transmission phase is a common mode signal. This signal is combined with the detected signal acquired during reception in a differential amplifier, whereby errors such as phase drifts due to the electrical circuits are canceled.
Both transducer transmissions occur at the same time and the received signals are phase compared against each other. Since the received signals are also received at the same time at zero flow rate, the flow sensing errors due to variances in the propagation of the acoustic energy are minimal. However, in another embodiment of the present invention, transmissions and receptions can occur alternately if desired, whereby each reception can, for example, be phase compared against a common reference signal. In a further embodiment, both transducers and phase detector are responsive to the transducer signals produced by the acoustic transmissions which are reflected back to the originating transducerto produce a common mode correction signal also responsive to acoustic path variations.